1. Field of the Invention
This invention relates generally to passive imaging systems with multiple apertures used to estimate three dimensional object positions.
2. Description of the Related Art
Conventional optical systems such as cameras or microscopes use imaging systems to detect two-dimensional images of three-dimensional objects. The dimensionality reduction from three dimensions to two dimensions results in a loss of information, usually a loss that is more prominent in the depth (z) dimension. While the transverse (x,y) dimensions of the object space enjoy close to a one-to-one mapping with the image space, the depth dimension is often reduced to more subtle cues such as defocus, parallax, and perspective, all of which together make it a difficult problem to quantify distances in the third dimension (i.e., to perform depth estimation).
Existing three-dimensional imaging techniques (i.e., imaging techniques that produce x,y,and z information) can be broadly divided into active and passive systems, depending on whether or not a predetermined light pattern or a light pulse is projected onto the object. Active systems, such as the XBOX 360 Kinect, use prior knowledge about the projected pattern to produce quantitative depth estimates. However, active systems require additional equipment to generate and project the light into the scene and they introduce to the scene additional light which is not native to the scene.
In contrast, passive systems produce depth estimates based on light that is naturally scattered from the object, such as room light or sunlight. Stereo-imaging systems are one example of a passive system. A stereo-imaging system acquires images of an object from two different views. The two images are used to triangulate the depth location of object points that appear in both views. In a typical stereo-imaging system, the two images are acquired by two cameras that are separated by a distance, which is referred to the baseline distance. In general, longer baseline distances yield more accurate depth estimates. The uncertainty in the depth estimate is approximately
                              δ          ≈                                    ɛ              ⁢                                                          ⁢                              Z                2                                                    i              ⁢                                                          ⁢              b                                      ,                            (        1        )            where ε is the uncertainty in the disparity estimation (i.e., the spatial uncertainty in matching a point in one image to the corresponding point in the other image), Z is the actual depth, i is the distance from the lens to the image (assuming a single thin lens model for the imaging system), and b is the stereo baseline distance. The depth uncertainty δ can be improved by increasing i, increasing b or decreasing ε. However, each of these generally will cause an increase in system size. Increasing i increases the distance from lens to image; increasing b increases the distance between the two cameras; and decreasing ε increases the lens size because E is inversely proportional to the lens diameter. Hence, reducing the depth uncertainty δ generally requires increasing the system size.
As a result, typical implementations of passive stereo-imaging systems have sizes that are at least of the order of several cubic centimeters. As described above, any reduction in size typically causes a corresponding penalty in performance. This size vs. performance tradeoff makes it challenging to design stereo-imaging systems that are both accurate and compact. The tradeoff is especially problematic if the entire imaging system with both cameras is intended to be implemented using a single sensor array, such as a CMOS or a CCD array sensor chip. These chips are bounded in size, with dimensions that often may be as small as a few square millimeters in surface area. This small size inherently limits the depth accuracy of any such stereo-imaging system.
Thus, there is a need for compact passive imaging systems that can also produce good depth estimates.